When you first go register to vote you have to provide driver's license or social security card or personal id or birth certificate which are things only citizens can obtain. Afterwards when you go to vote they check your name against a list. You sign a card saying you're you and vote..
Not really, I'm not a citizen, but when I came there, I got a social security card and a driver's license. So short of an american birth certificate and an american passport, you could get any other form of ID and not be a citizen. I imagine when you register, they run down your information and social security number to see if you are a citizen or not.
"Fire cannot kill a dragon." -Daenerys Targaryen, Game of Thrones
When you first go register to vote you have to provide driver's license or social security card or personal id or birth certificate which are things only citizens can obtain. Afterwards when you go to vote they check your name against a list. You sign a card saying you're you and vote.. 
